1. Field of the Invention
The present invention relates to a protective circuit for protecting an internal circuit against an abnormal voltage or abnormal current such as static electricity or a surge, and also relates to an electric circuit using the protective circuit.
2. Description of the Related Art
A conventional protective circuit protects an internal circuit in such a manner that the protective circuit is connected between a ground terminal and external input/output terminals extending from the internal circuit, and is turned on only in the case where static electricity or a surge is applied, so that the static electricity or surge flows from the input/output terminals to the ground terminal without flowing through the internal circuit.
FIG. 2 is a block diagram showing an electric circuit including a conventional protective circuit.
In FIG. 2, a ground terminal 205, an external input terminal 206 for connecting a power source 208, and an external output terminal 207 for connecting an external load 209 are extended from an internal circuit 203 to which a specific function is given according to an object. A protective circuit 201 is connected between the ground terminal 205 and the external input terminal 206, and a protective circuit 202 is connected between the ground terminal 205 and the external output terminal 207, so that an electric circuit 204 is formed.
When a voltage higher than the maximum operation voltage of the internal circuit is applied to the external input terminal 206, the protective circuit 201 is immediately turned on to permit a current to flow therethrough, so that the voltage at the external input terminal 206 is lowered. In this way, the protective circuit protects the internal circuit 203 against static electricity or a surge applied to the external input terminal 206. The protective circuit 202 also protects the internal circuit 203 against static electricity or a surge applied to the external output terminal 207 in the same way as the protective circuit 201.
FIG. 3 is a circuit diagram showing a conventional protective circuit.
In FIG. 3, a protective circuit 301 is formed in such a manner that a drain of an enhancement N-channel MOS transistor 302 (hereinafter referred to as NMOS Tr 302), which is one type of MIS transistor, is connected to an input terminal 306, and a gate, source, and substrate are connected to a ground terminal 205. Since gate and source of the NMOS Tr 302 are of the same potential, a channel between the drain and source is normally in an insulated state.
The protecting operation of the protective circuit shown in FIG. 3 will be described with reference to FIG. 4.
FIG. 4 is a diagram showing an I-V curve of the protective circuit 301.
In FIG. 4, when a voltage applied between the input terminal 306 and the ground terminal 205 of the protective circuit in FIG. 3 is increased, a surface breakdown is caused in the NMOS Tr 302, which has been in an insulated state, at the time when the voltage exceeds the withstand voltage Vt between the drain and source, and further a snapback is induced through a hold voltage Vh. As a result, the channel between the drain and source is made conductive. By this, a large current flows from the input terminal 306 to the ground terminal 205 through the NMOS Tr 302 as the protective circuit 301, so that the voltage at the input terminal 306 is lowered. Thereafter, when the voltage at the input terminal 306 falls below Vh, the NMOS Tr 302 returns again to an insulated state and the protective circuit 301 ends the protecting operation.
When the withstand voltage Vt of the NMOS Tr 302 is adjusted so that it is not less than the maximum operation voltage of the internal circuit 203 and not larger than the withstand voltage of the internal circuit 203, and the input terminal 306 is replaced by the external input terminal 206 or the external output terminal 207, the protective circuit 301 in FIG. 3 can be used as the protective circuit 201 or the protective circuit 202 in FIG. 2, and can protect the internal circuit 203 against static electricity or a surge input to the external input terminal 206 or the external output terminal 207.
However, in the conventional protective circuit 301 constructed as in FIG. 3, since a current at the protecting operation is restricted by only a small conduction resistance of the NMOS Tr 302, a large current is apt to flow through the protective circuit 301. In addition, since it is difficult to increase the conduction resistance, and further, the unevenness in manufacture of semiconductor is large, there is a problem in that it is difficult to obtain a target value and to adjust the resistance.
In FIG. 4, when a voltage applied to the input terminal 306 is further increased from the state in which a large current flows by the snapback, if a current increases so that the current exceeds an allowable current Im of the NMOS Tr 302, the NMOS Tr 302 causes heat destruction so that it enters a short circuit or insulating state and is destroyed. In the case of short circuit destruction, since the input terminal 306 and the ground terminal 205 are short-circuited, it becomes impossible to apply an input signal to the input terminal 306, and in the case where the internal circuit is connected to the input terminal 306, the internal circuit becomes impossible to operate normally. On the other hand, in the case of insulation destruction, since the protective circuit 301 does not operate thereafter even if a surge is applied to the input terminal 306, in the case where the internal circuit is connected to the input terminal 306, there is a case where the surge is applied to the internal circuit so that the internal circuit is destroyed. This, the destruction of the protective circuit has a considerably bad effect on the internal circuit and the electric circuit consisting of the protective circuit and the internal circuit.
In the conventional protective circuit, since the resistance during the protecting operation is small, there is a problem that a current over an allowable current of a MIS transistor is apt to flow so that the destruction of the protective circuit as described above is apt to occur. Moreover, there is a problem in that malfunction or destruction of the internal circuit caused by the destruction of the protective circuit is apt to occur. That is, there is a problem in that the protective limit against a surge is low so that sufficient protective characteristics can not be obtained.
In addition, in the conventional protective circuit, there is a problem in that since it is difficult to increase and adjust the resistance during the protecting operation, it is impossible to freely adjust the protective limit.
With the above-mentioned problems, there is a problem in that it is impossible to provide an electric circuit having improved safety.